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simulink matlab function block error

Asked by Zhengyuan Wang on 4 Aug 2015
Latest activity Edited by Mike Hosea
on 4 Aug 2015
Hello guys,
Im trying to calculate my lqr gains online, im having some troubles with the simulation, i get the error message saying
Function output 'Kr' cannot be an mxArray in this context. Consider preinitializing the output variable with a known type.
Errors occurred during parsing of MATLAB function
Here is the code, I am trying to obtain the Kr(1,1) in this case.
% code
function Kr = fcn(u)
%#codegen
eml.extrinsic('lqr');
a = 1.3629;
b = 1.2996;
Cf = 16000;
Cr = 16000;
Cr2 = 15000;
I = 2761;
l = a+b;
m = 2024.65;
w = 2e-7;
a11 = -2*(Cf+Cr)/(m*u);
a12 =(2*(b*Cr - a*Cf)/(m*u))-u;
a21 = 2*(b*Cr-a*Cf)/(I*u);
a22 = -2*(b^2*Cr+a^2*Cf)/(I*u);
b1 = 0;
b2 = 1/I;
A = [a11 a12; a21 a22];
B = [b1; b2];
Q = [0 0; 0 1];
R = [w];
Kr = lqr(A,B,Q,R);
end

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2 Answers

Answer by Mike Hosea
on 4 Aug 2015
 Accepted Answer

You need to tell the compiler what the output of lqr will look like.
Kr = zeros(1,2); % Define the output type for the extrinsic call.
Kr = lqr(A,B,Q,R);
Then you can do what you want with Kr.

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Answer by Walter Roberson
on 4 Aug 2015

Change it to
tKr = lqr(A,B,Q,R);
Kr = tKr;

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Show 1 older comment
Walter, why did you suggest that?
Because the usual reason that this error occurs is that you have called a MATLAB routine and passed the output directly onwards to another routine, and the usual fix is to assign the value to a variable and use the variable.
Thanks. I understand where you were coming from now. The tidbit that you're missing is that this variable needs already to have been defined (and to have the right type and size). It can't appear first as the output of an extrinsic function. The compiler will use what it knows about the size and type of the variable to copy the data out of the mxArray into "native" data types before continuing on.

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